Sections 1, 2, and part of 3, as listed above, deal with the MHC-I aspects of this project, and in general are directed to understand the molecular details of the loading of MHC-I molecules with self or antigenic peptides. Part 1 has resulted in a detailed understanding of the three-dimensional structure and structural changes that accompany MHC-I interaction with its chaperone, TAPBPR, and with the release of the chaperone from the MHC-I molecule on interaction with peptide. Current experiments are underway to explore the MHC-I interaction with tapasin, the crucial chaperone of the peptide loading complex (PLC). In Part 2 of the studies of MHC-I/peptide interactions, we have explored the role that the anti-retroviral drug, abacavir, plays in binding to MHC-I and distorting the self-peptide repertoire bound by susceptible MHC-I alleles. In particular, we examined the biological effects of abacavir binding to HLA-B*57:01 in a model animal system that we have developed. Thus, abacavir alone, when bound by HLA-B*57:01 in a transgenic animal, can elicit a neoantigen T cell response, dependent on regulation of CD4 T cells. This animal model system provides an explanation for the severe hypersensitivity reactions that are observed in a high proportion of HLA-B*57:01 individuals who receive the drug. The initial studies were performed with HLA-B*57:01 transgenic animals on a normal mouse MHC background. We are now exploring the ability of abacavir to be presented in various MHC deficient backgrounds. The third part of this project is focused on structural and functional studies of T cell receptor recognition of antigens, how this leads to T cell signaling, and how this leads to autoimmune disease. Finally, in Part 3, to provide a baseline for understanding antigen-specific structural changes in the T cell receptor (TCR), we have determined the X-ray structure of a virus specific, MHC-I-restricted TCR, as well as its complex with its MHC-I/viral antigen ligand. Remarkably, although the MHC/peptide complex has a relatively rigid structure, the TCR shows great movement of its CDR3 alpha and beta loops, indicative of a fly-casting mechanism for ligand engagement. Further characterization of this fly-casting mechanism is reflected in other projects. Collaborative NMR experiments have examined the details of the effects of the chaperone protein TAPBPR in catalyzing the selection of high affinity peptides for cell surface expression.